Sustainable Stormwater Management for Different Types of Water-Scarce Cities: Environmental Policy Effect of Sponge City Projects in China

Abstract

With high-speed urbanization, ecological space is seriously shrinking, and lagging drainage facilities contradict the ecological needs of citizens. In particular, water-scarce cities are faced with frequent stormwater disasters, such as excessive accumulation of rainwater, peak runoff and water pollution, which threaten the safety of the urban water ecological environment. This paper combined the actual construction content of the sponge city project with a whole process policy evaluation framework to examine whether the projects solve these problems and to find different approaches to the results. Utilizing entropy fuzzy comprehensive evaluation provides a systematic standard for the evaluation system. The research shows that the sponge city project can achieve a good governance effect, including constructing a suitable scheme for urban hydrological characteristics, effectively improving the rainwater treatment level of different types of water-scarce cities, and alleviating the ecological contradiction of urban water environment. The stages of policy formulation, policy implementation and policy results achieve a good degree of completion. On one hand, sponge city projects transform the infrastructure at key locations, aiming at improving the rainwater interception capacity of the streets; on the other hand, restoring original natural waters improves the capacity of water conservation and forms a sustainable ecosystem between the city and nature.

1. Introduction

With rapid urbanization, cities around the world have been suffering stormwater issues for a long time. Extreme rainfall events caused by climate change bring urban floods and destroy everything in the place where they occur, resulting in huge economic losses [1]. According to a report published by WMO in 2021, over 11,000 disasters were reported worldwide in the period 1970–2019, causing more than 2 million deaths and USD 4.3 trillion losses [2]. Global stormwater issues also have caused trouble in China. According to the “China Flood and Drought Disaster Prevention Communique 2020”, the direct economic losses caused by floods exceeded 250 billion yuan in 2020 alone. The Yangtze River and Taihu Lake basin were hit by major floods, and the Huaihe River and Songhua River were hit by large regional floods, which occurred simultaneously for the first time since 1998 [3]. Serious flood situations and wide-scope disasters make the relationship between men and nature no longer harmonious, and the balance between urban development and the environment has also been broken. The underlying surface formed by asphalt pavement and cement building roofs has become much harder, resulting in 70–80% of the city’s precipitation turning into runoff. Only 20–30% of the stormwater directly penetrates into the ground, destroying the urban ecological ability to absorb rainfall and damaging the role of natural “sponge” [4]. Serious urban stormwater problems have occurred, with severe and rapid conversion of droughts and floods and other urban water environment problems caused by traditional urban construction. Water ecological destruction and water resource shortage make the water environment in urgent need of management, seriously threatening citizens’ water life safety.

To achieve sustainable stormwater management, the sponge city project has been put forward in China, drawing upon lessons from other advanced stormwater management techniques. Sponge cities, a concept gaining popularity worldwide, aim to mimic natural hydrological cycles by enhancing urban drainage systems. As an important way to restore the urban water ecological environment, they can not only solve the problems of urban drought and serious shortage of water resources supply but also use the LID system to absorb, store and purify rainwater and adjust the balance of the urban water ecological environment [5,6]. To achieve comprehensive ecological benefits, different unit combinations of LID technology are applied in the construction of a sponge city. It is concluded that biological detention facilities, wetlands, sinking green space and permeable pavement could all better realize ecological service functions and naturally manage urban stormwater from the source [7]. However, despite their promise, these projects face challenges such as limited urban space, funding shortages and technical difficulties. The development of green buildings lacks the support of space and effective policy documents, though grey infrastructure still plays an important role in the construction of sponge cities [8]. As for the funding source, WTP (willingness to pay) is used as a tool to explore the behavioral intention of local people and the comprehensive benefits of the project [9]. It is found that the public is willing to pay a certain degree of water price surcharge, having faith that the PPP model would provide the main funding for sponge city projects [10]. The lack of short-term arrangements and sustainable stormwater management knowledge limits the promotion of sponge city projects across the country [11].

This study aims to explore how sponge city projects can effectively address urban drainage issues, especially in water-scarce regions. Due to excessive groundwater exploitation and severe water pollution, many cities have been faced with serious urban water ecological problems, such as so-called water-scarce cities [12]. They are threatened by severe droughts in the dry season and heavy rain in the rainy season, which has forced the interruption of urban economic activities and brought huge losses and negative social impacts. It is found that urban stormwater management in water-scarce cities is related to the urban ecological environment. The loss of sustainable urban stormwater management has hindered water-scarce cities from building urban ecological systems and the sponge city project has been put forward to solve these problems [13]. Combined with advanced water-control technology like LID and WSUD, the project is supposed to improve water ecological effects, which may not only solve the serious problem of urban stormwater management but also achieve a full range of urban ecological benefits [14]. Thus, the environmental and social impacts of sponge city are beneficial and financially feasible, and comprehensive long-term planning for such projects needs to be conducted to achieve sustainable stormwater management and protect the urban ecological environment.

The margin contribution of this study is mainly about the evaluation of sponge city projects referring to the adaptability of different urban hydrological characteristics. (1) Based on the main water shortage characteristics of Chinese cities, we formed a whole process framework in different policy life cycles, providing a systematic evaluation standard for the environmental policy effect produced by sponge city projects; (2) we conducted an empirical evaluation on the water environment effect of sponge city and examined whether these projects would solve problems for water-scarce cities; (3) we analyzed different approaches to sponge city projects according to the completed construction results. The structure of the paper is divided as follows. In Section 2 “materials and methods”, we started with problems of limited urban stormwater management and the benefit of sponge city projects, to contextualize the topic and put forward the theoretical framework of a comprehensive evaluation of urban stormwater management and water ecological environment, and then we introduce the project context with main characteristics of two types water-scarce cities in China, to design specific indicators for the evaluation system with a fuzzy comprehensive model of entropy method. In Section 3 “results”, we analyzed the results of the evaluation of these sponge city projects, including policy formulation, policy implementation and policy results, discussed how to realize the policy effect and possible reasons for variations in project effectiveness across different cities. In Section 4 “discussion”, we made a discussion about different approaches to sponge city projects and compared the strengths and weaknesses, including improvement measures. In Section 5 “conclusions”, we provided a comprehensive summary of the findings, emphasizing the importance of sponge city projects in addressing urban water environmental pressures and improving water use efficiency, limitations of the research and recommendations for future work.

2. Materials and Methods

2.1. Literature Review

Urban stormwater management is one of the important duties of local government. However, it requires the cooperation of various stakeholders to attain sustainable development. The governance of urban water environment security could be divided into adaptive governance, polycentric governance, social learning and multi-level governance [15]. The government should cooperate with social organizations in controlling urban stormwater based on institutional trust [16]. Also, the government is still the dominant player at present, with network and polycentric management in the early developing period [17]. It is proposed that the construction of water environment governance projects of the life cycle from the perspective of enterprises is to meet the water environment demand and tap the value of urban development [18]. As for the governance effect and evaluation of urban water environment policies, data envelopment analysis (DEA) is applied to develop a new environmental policy evaluation strategy to deal with the overestimation of environmental policy efficiency [19]. It is found that the policy combination of environmental legislation, ecological compensation and environmental protection negotiation has a good effect on environmental pollution control [20]. To motivate local officials to actively deal with environmental problems, there is a good way to change the promotion standards, adding an environment-related index to the performance evaluation [21]. The establishment of sustainable stormwater management requires the government to play an important role and to motivate all stakeholders in the process, with a long payback period for environmental projects.

The lagging of rainwater treatment facilities in water-scarce cities hinders the sustainable development of urban stormwater management, which is caused by an economic-growth-dominated urban development model and the lack of a planning orientation that combines environmental protection and development. Inadequate or poorly maintained urban drainage networks and outdated design of rainwater treatment standards do not meet the requirements of urban population growth and climate change [22]. The loss of sustainable urban planning makes water-scarce cities vulnerable to the rainy season. In more than 90% of these cities, the design of facilities to deal with urban stormwater is linked with traditional engineering infrastructure with much cost and inflexibility [23]. Climate change is the driving factor for the increase in flood disasters in China [24]. Supporting studies have shown that more extreme events have occurred in recent years, especially high-intensity rainfall, and the average annual number of heavy precipitation days has also increased significantly [25]. Extreme precipitation accounts for about 1/3 of the annual rainfall, and the rate is much higher than previously estimated [26]. The frequency and intensity have increased the probability of flood disasters in terms of hydrological conditions, and urbanization has increased the possibility of exposure to flood disasters. Moreover, the impervious surface area of cities has been steadily increasing at a rate of 6.5% per year [27]. The continuous loss of the urban water ecosystem, including the fragmentation trend of urban waters, has seriously reduced the ability of urban aquatic landscape to absorb, store and permeate precipitation, and intensified the urban runoff and soil pollution, which has seriously threatened the balance and security of urban water ecological environment. Thus, it is indispensable to put forward holistic and systematic urban development and management methods to enhance the resilience of water-scarce cities, to establish sustainable stormwater management and to solve water ecological problems.

The sponge city project has been into implementation under such circumstances, aiming at solving the contradiction between urban stormwater issues and water shortage problems, and promoting urban water ecological benefit. The “Sponge City Construction Guide” and “Guiding Opinions on Promoting the Construction of Sponge Cities”, two fundamental documents in sponge city project have provided guidance for the top-level design of construction of sponge city projects from both policy and technology levels, the former issued by MOHURD in 2014 and the latter by State Council in 2015. In these official government documents, a sponge city is defined as a form of urban development that makes full use of the absorption, penetration and detention functions of ecosystems such as buildings, roads, green spaces and waters to achieve the control of urban rainwater runoff and natural storage and purification. More importantly, it is proposed to solve the following problems. The first one is to protect the original ecological environment of the city, to restore the polluted waters and to increase the proportion of natural waters. The second one is to apply LID technology to water resources management and to utilize different stormwater management strategies. Some specific technologies used in the construction of sponge city are composed of three interdependent parts: LID measures, pipe drainage system and excessive rainwater runoff drainage system. The goal of sponge city construction requires that urban stormwater issues be gradually shifted to the problem of the water environment of the city [28,29].

Some cities which have already begun to construct the projects are called “pilot cities”, and it is found that the promotion of the project was top-down and government-led, with less public participation. The practice of the pilot cities could effectively expand the influence of the project, but the tight time arrangement hindered the achievement of the effectiveness [30]. It is also believed that the practice of sponge city projects faces various challenges, and it is suggested to build a city-to-city learning mechanism, participatory comprehensive collaborative governance initiative and to create favorable conditions for investment [31]. The construction of sponge city is believed to alleviate the urban heat island effect and control urban water-logging, including the LID (low-impact development) model providing space for urban green buildings [32]. Also, it contains the environment-friendly connotation of low-impact development mode and sustainable urban construction, and the development of sponge cities has an internal connection with the construction of ecological, low-carbon and smart cities [33].

2.2. Theoretical Framework of Policy Effect Evaluation for Sponge City Project

This paper is about using a policy evaluation model to measure the effect of the sponge city project on stormwater management and the development of an urban water ecological environment. Projects with feasible policy arrangements driven by the government, are usually carried out by PPP (public-private partnership), which is necessary to build a decentralized governance structure in order to achieve cross-departmental collaboration [34]. In terms of different stages of the policy process, the policy evaluation model mainly includes policy formulation evaluation (prospective prior evaluation), policy implementation evaluation (monitoring mid-term evaluation) and policy results evaluation (retrospective post-evaluation), to systematically analyze the content, implementation and impact of policies. It also examines disadvantages and enhances the economic value and social benefits for stakeholders [35]. At present, policy evaluation is generally regarded as a whole policy process, a life-cycle process covering policy formulation, implementation and results. It is because of urban stormwater issues and water ecological problems that sponge city projects have become the policy arrangements.

The construction of a sponge city improves the urban water cycle system both at the micro basin and macro scale. Based on the natural environment, it would promote the resilience of urban waters and enhance the connectivity between community and region, while avoiding the subdivision and isolation of different basins [36]. At the micro-scale, it continues to control urban runoff during extreme rainfall with engineering infrastructure, treating rainwater at key locations, such as artificial wetland parks, community green spaces and other catchment units [37]. At the medium scale, the focus is on integrating micro-projects with nearby waters and following urban stormwater management projects such as peak runoff, runoff pollution, and rainwater reuse [38]. On a macro level, sponge city would establish a regional green infrastructure network, which could protect rivers, wetlands and other urban waters, achieve comprehensive water resources protection and promote an urban green landscape [39]. The integration of micro-scale and medium-scale sponge projects into a system would provide opportunities to solve the urban stormwater management issues and ecological problems of water-shortage cities and to effectively improve the regional urban water ecological benefit. Based on the model and relevant hydrological conditions, the theoretical framework of comprehensive evaluation of stormwater management and urban water ecological environment is constructed as follows (Figure 1).

2.3. Main Characteristics of Two Types of Water-Scarce Cities and Project Context

The types of urban water shortage in China mainly include resource-scarce type and poor water quality type. The former, water resource-scarce cities are mainly located in the North China Plain, Northwest China, some parts of Northeast China and the middle reaches of the Yellow River [40]. Due to the lack of surface water systems, over-exploitation of groundwater, and perennial interruption of river flow, such cities are threatened by severe drought in the dry season and by heavy rains in the rainy season, bringing great loss and negative social influence. According to a survey by the Ministry of Housing and Urban–Rural Development (MOHURD), 62 percent of the 351 cities suffered urban inundation floods and 39 percent experienced more than three major floods between 2008 and 2010 in China. The number of cities affected by floods has more than doubled since 2008. Almost every year, at least 130 cities are hit by urban stormwater issues [41]. Since the GDP of cities accounts for more than 80% of the country and the population accounts for more than 50% of the total population, the destruction impact on both the economy and society has increasingly become a great problem of China’s urbanization development [42].

Poor water quality type cities are caused by the pollution of the water source, making the quality lower than the standard of industrial and domestic water. Such cities are mainly distributed in highly industrialized, economically developed areas and large-sized urban agglomerations, concentrated in the Yangtze River, Huaihe River and Pearl River basins. In these urban agglomerations, the water quality of some small and medium-sized cities continues to hover at the edge, and the problem of insufficient water has existed for a long time. Urban development is usually dominated by economic growth and lacks a planning orientation that combines ecosystem protection with development. Inadequate or poorly maintained urban drainage networks and outdated design of rainwater treatment standards do not meet the requirements of urban population growth and climate change. Unsustainable urban planning makes cities vulnerable to the rainy season. In more than 90% of cities, the design of facilities to deal with urban stormwater is linked with traditional engineering infrastructure, which is costly and inflexible. Although urban sewage can be quickly discharged downstream of the water area, it is unable to cope with heavy rainfall events, resulting in the leakage and discharge of pollutants directly into the urban rivers and the degradation of the water area.

This paper has chosen two cities in China, one located in the northern part and the other one in the southern. Located in the center of Northeast China, the S city has distinct four seasons, belonging to the semi-humid and semi-arid regions of the eastern monsoon. The annual average rainfall is 604.2 mm, with a maximum of 1008 mm every year and a minimum of 404 mm. The precipitation is unevenly distributed in the year, which accounts for 77.8% in the summer of the whole year. The average monthly maximum and minimum temperature and average monthly rainfall (between 2011 and 2020) are shown in Figure 2 and Figure 3. The annual average evaporation is between 750–1000 mm, making it one of the dry areas in the province [43]. The city has few surface waters, with groundwater mainly recharged by atmospheric precipitation and lateral seepage of rivers. The evaporation and artificial exploitation of waters are the main drainage ways. The main rivers flowing through the city are the South River, the North River and its tributary. Water supply is mainly from groundwater, accounting for more than 70%, and more than half of the supply is used for agriculture [44]. Due to the hydrological conditions and rapid urban development, S city is faced with a severe water resource shortage. Problems can be summarized as follows: (1) the flow of surface waters is too little to meet the ecological requirements, some even in a state of cut-off; (2) a large number of agricultural planting areas lack isolation and protection, so residual pesticides and fertilizers are directly discharged into the river with rainwater, resulting in non-point source pollution; (3) the runoff in the rainy season carries a large amount of sediment into the river channel, leading to a large number of aquatic plants and weeds eroding the channel and weakening the flood discharge capacity of the channel; (4) the sewage treatment plant lacks in-depth design and reuse plan of recycled water, leading to the waste of water resources.

Located in the warm and humid subtropical monsoon climate, the C city has mild temperatures, mild rainfall, and a long plant growth period. The annual precipitation is between 1400–1590 mm, most years more than 1000 mm. The average monthly maximum and minimum temperature and average monthly rainfall (between 2011 and 2020) are shown in Figure 4 and Figure 5. Its landform type is more complex, the whole terrain gradually declines from southeast to northwest, from Zhongshan mountains, low mountains to hills, and finally to hilly land and plain [45]. The rapid development of the economy and the increase in population brought water pollution to the city, with the quality of urban waters classified as poor fifth. Problems can be summarized as follows: (1) The water quality of Qingxi River in the central city is relatively poor, and the water quality monitoring data show that the water quality ammonia nitrogen pollution is serious, in which TN (total nitrogen) and COD (chemical oxygen demand) are the main factors exceeding the standard. (2) There are black and smelly water bodies in two natural lakes. Due to the imperfect pipeline network of the surrounding land and the illegal discharge of sewage, the water bodies of Guanhu and Zhaowei have been seriously polluted by point sources and non-point sources, which not only affect the normal life of the surrounding residents but also lose important ecological functions. (3) The runoff pollution is serious, and the spillover pollution of the combined flow system is prominent. There is also the phenomenon that the overflow on a dry day enters the river directly through the combined drainage system, and the problem of mixed or incorrect connection of rain and pollution is widespread [46].

Although these two cities have different hydrological conditions and problems with water resources, the rapid development of the economy and the increase in population brought similar stormwater treatment problems to the cities. Due to the expansion of urbanization, the impervious layer in built-up areas has expanded, bringing the water supply and drainage system great pressure. Thus, the increasing pressure of sewage discharge treatment in the main urban area makes the urban living environment deteriorate continuously. Urban stormwater management is faced with serious challenges, which are mainly concluded in the following aspects. (1) The design of drainage system capacity is insufficient to cope with urban hydrological changes. The service period of the urban drainage pipe network is more than 50 years, during which at least three times of urban planning revisions will be experienced, so there are many uncertainties. Most of the pipe network service life is too long to provide regular maintenance, resulting in poor flow and other problems. (2) The construction of the drainage system is relatively lagging behind with some pipelines in disrepair. The roads in the central city mostly use the combined drainage system, that is, rainwater and sewage are collected by the same drainage system and discharged into the natural waters, resulting in a large number of industrial and domestic wastewater pollution. (3) Rainwater absorption capacity is not enough to cope with urban stormwater. With the construction of the new city area and the reconstruction of the old city area, some urban waters such as pools and ponds are buried, resulting in the loss of stormwater absorption space. The total amount of water has reduced, only left parks playing a dominant function. Directly, rainwater quickly collects into the pipe network and increases the burden of the drainage system.

Based on the problems mentioned in urban water ecology, the municipal governments initiated the preparatory work of the sponge city project. Due to the long investment cycle of environmental urban infrastructure projects, slow return on capital, and the need for a large amount of financing, the municipal government decided to launch the project as PPP. After successfully bidding for social capital, the corporation was set up. The capital source is composed of three parts: the government’s own funds, social capital and loans from financial institutions. The operation mode of “investment, construction and operation integration + payment for recycled water reuse + feasibility gap subsidy” is adopted. The Housing and Construction Bureau of the city signs the PPP Contract with the corporation according to the authorization of the government of the city and grants it the franchise right (Figure 6). Upon expiration, the facilities of projects shall be handed over free of charge to the authority or other designated agencies in good condition as required.

2.4. The Design of Policy Evaluation System and Specific Indicators

An evaluation system is to formulate and implement the inspection of a policy and what kind of results are produced, of which the key is to design appropriate indicators. Using a life cycle perspective to evaluate the outcome of a policy, would provide valuable information, examine the factors that promote or hinder policy implementation and further improve inefficient policies to support management decisions. The overall process can be divided into three interactive policy parts, including policy formulation, policy implementation and policy results [47].

Firstly, the policy formulation dimension mainly includes three parts: objective, tool and system. The objective part refers to the fundamental logic contained in the policy formulation, whether it meets the conditions and could be feasibly formulated according to the current reality. Also, it includes comprehensiveness, covering all stages of the sponge city project, detailed formulation of specific strategies, and full consideration of the balance of interests of different stakeholders [48]. Tools include the diversity of the tools needed for policy formulation, to play a good role in implementation and solve problems effectively. The system refers to the integrity of policymaking, and the policy system covers different types, such as laws, plans, guidelines and standards.

Secondly, it is the process of policy implementation, which mainly includes two parts: capacity and qualification. The capacity of policy implementation mainly refers to the ability of local governments, like water authorities, urban construction and other relevant departments, to implement the construction of a sponge city project. To make it more detailed, the capacity is to activate capital, deploy manpower, promote advanced technology, document and share project progress, encourage public participation, handle public feedback, and clarify stakeholder responsibilities. The relevant elements of the qualification of the policy implementation include the investigation and identification of environmental values and the assessment of environmental risks and human settlement health risks.

Thirdly, the policy result dimension includes three parts: the achievements of policy implementation, the degree of policy response and the efficiency test. The achievements of the policy include the improvement of the environmental condition of selected sites and the economic benefits generated by the use of the facilities. Traditional piped drainage systems can easily calculate investment and performance, while urban stormwater management systems based on green infrastructure need to set new standards to measure [49]. Responsiveness refers to the public’s awareness improvement and active participation in the projects. The efficiency includes the evaluation of subsequent resource input, capital stability and overall effectiveness of the project.

The purpose of this evaluation process is to explore how sponge city policies were developed, the results of implementation, and to achieve the objectives by combining the role of sponge city policies in building sustainable stormwater management and bringing urban ecological benefit. On the one hand, sponge city projects bring sustainable stormwater management by applying more LID technology in the construction, such as more gullies, infiltration channels and permeable pavement to increase urban rainfall penetration and storage. The public knows about urban stormwater disasters and the ecological value of the projects through government publicity, making it a platform for communication and exchange of sustainable stormwater management [50]. The committee would plan to design appropriate indicators of rainwater control such as runoff, peak runoff, runoff pollution, and rainwater utilization. The most important standard is the volume capture ratio of annual rainfall, which means that the percentage of rainwater captured through infiltration, storage and evaporation in the total annual rainfall of the design site meets a fixed range. Although the old urban area and the newly built area need different solutions in the same city, stormwater disasters can be solved by effectively controlling urban rainwater, giving full play to the roles of buildings, roads and urban water systems [51]. It is indispensable to use computer technology as an auxiliary design tool in rainwater absorption, infiltration and storage, constructing rainwater management systems and making aquatic systems conducive to the sustainable development of urban water environment.

On the other hand, sponge city projects improve urban water ecological benefit by protecting rivers, wetlands and other urban waters, correspondingly increasing urban green space, such as vegetation areas, forests and farmland. The constructed wetlands in urban parks also improve the capacity for carrying, conservation and purification of urban waters [52]. However, the increase in urban density makes the space of public land less and faces the key problem of changing old gray infrastructure into green infrastructure and adding some new, so the effectiveness of space utilization has become a difficulty [53]. Urban landscape architects should play a leading role, identify environmental values, combine hydrological conditions with urban planning, and form localized solutions to avoid weak coupling problems arising from the construction experience of other areas [54]. After the completion of green infrastructure, fees can be charged to improve the effectiveness of its management, though the ecological functions of green facilities need enough time to play and experience. Thus, different elements of the city are combined into a unified overall ecosystem. Through the source control of urban waters, the precipitation that originally caused pollution and stormwater burden is transformed into an effective resource conducive to the repair of the urban water ecological environment, which can not only realize the intensive utilization of water resources but also become an effective way to promote urban green development.

Based on the theoretical framework and the principles of simplicity, representativeness and comprehensiveness, this paper conceptualized 16 evaluation indicators and designed 35 specific questions, with reference to the procedures of environmental policy analysis and the construction and effect of sponge city projects [55]. It is designed to cover the whole policy life cycle and to evaluate what results sponge city projects have achieved at multiple levels. By combining the policy evaluation model from the perspective of the life cycle with the actual construction objectives of sponge city projects, the specific indicators are obtained as follows (see

Table 1. Indicators of the evaluation system.

Policy Evaluation	Component	Indicator	Number	Description
Policy formulation	Objectives	Reliability	Q1	Achieve sustainable urban stormwater management
			Q2	Improve the urban water ecological environment
		Comprehensive	Q3	Cover both political and environmental cultural connotations
		Feasibility	Q4	Plan and design objectives to meet the basic stormwater control requirements
			Q5	Build the required stormwater management model and use assistive technologies
			Q6	Have detailed technical guidelines
			Q7	Develop an operational timetable
	Tool	Diversity	Q8	Have both mandatory and voluntary tools
	System	Integrity	Q9	Cover different types of documents (e.g., regulations, guidelines, standards, etc.)
Policy implementation	Capacity	Allocation of resources	Q10	Funds can be rationally allocated to different projects
			Q11	Professionals can be assigned to appropriate jobs
		Sharing of professional knowledge and information	Q12	Inspect site conditions, record relevant materials and share with other departments
			Q13	Participate in the training of rainwater management knowledge
			Q14	Build an effective stakeholder information-sharing network to Carry out social publicity and education activities
		LID facility construction	Q15	Activate and utilize the original grey infrastructure
			Q16	Build suitable green infrastructure
		Coordination arrangement	Q17	Build a platform for subjects to interact
	Qualification	Identification value	Q18	Identify the environmental value of specific locations
			Q19	Fully perform ecological function of the original waters
		Assess risks	Q20	Assess environmental and human settlement risks
			Q21	Assess the risk of available funds and land space
Policy results	Achievements	Improved environmental conditions	Q22	Controlled water volume, reduced runoff and delayed peak value
			Q23	Improved water quality and reduced contaminants
		Improved economic efficiency	Q24	Stable output of investment in sustainable rainwater management facilities
			Q25	Reduced facility maintenance costs
	Response to	Public awareness	Q26	Improve awareness of citizens through publicity and education activities
			Q27	Form a collective awareness of water ecological protection
		Public participation	Q28	Actively contribute suggestions for the construction
	Efficiency	Stability	Q29	Stable investment in construction and subsequent restoration personnel
			Q30	Stable facility performance
			Q31	Sustainable capital flow and maintenance scheme
		Effectiveness	Q32	Comprehensive performance appraisal
			Q33	Realize urban water resource recycling
			Q34	Enhance the environmental value of urban aquatic landscape
			Q35	Realize the ecological function of urban waters

):

2.5. Data Collection and Fuzzy Comprehensive Model of Entropy Method

Data were collected through surveys distributed to cities implementing sponge city projects. These surveys focused on 35 indicators above such as water retention capacity, runoff reduction, and overall environmental impact. The collected data were then analyzed using a fuzzy comprehensive model of entropy method to determine the effectiveness of these projects.

The evaluation index assignment method of the fuzzy comprehensive evaluation model is based on the Likert scale, which is the widely used method to measure double-phase reactions in social surveys. In order to measure the comprehensive evaluation of stakeholders on a symmetric agree–disagree scale, a typical Likert five-level item is adopted, with responses set as strongly agree, agree, neutral, disagree, and strongly disagree and values of 1, 2, 3, 4, and 5 assigned to each level in turn. Some participants were invited through the Internet, a WeChat small program named “questionnaire star”, and others filled out the questionnaire of evaluation indicators during an academic conference about public service and management, to better understand the views of different subjects on the comprehensive evaluation of the effects of the projects. Thus, the degree of agreement of the evaluation indicators was made clear, including people from the government departments, research institutions and related industries in the selected cities. Questionnaires were distributed twice around July and November 2023, and a total of 168 valid questionnaires were collected from S city and C city. The basic information of interviewees is shown in

Table 2. General information related to participants.

Questions	Options	Percent
Sex	Male	56.2%
	Female	43.8%
Age	18–25	12.3%
	26–33	31.2%
	34–42	40.8%
	43–50	15.7%
Industry	Government	21.5%
	Construction	23.3%
	Education/research	30.5%
	Environmental protection	24.7%
Education	Bachelor	33.5%
	Master	41.9%
	PhD	24.6%

.

Some scholars have used the analytic hierarchy process to evaluate the comprehensive benefits of the urban water environment in Wuhan, comprehensively analyzing the positive externality from perspectives of ecological, economic and social benefits [56]. The fuzzy comprehensive evaluation model of the entropy method realizes value judgment by combining subjective expert experience, uses available objective data to realize comprehensive evaluation, and provides a systematic method for multi-criteria environmental policy assessment. According to the design of indicators, this evaluation model can be constructed according to the following steps:

The first step is to construct the set of indicators and value levels. The former is represented by U and the latter is by V, respectively, with 35 indicators and 5 levels.

U = {u₁, u₂, …, u_m}

(1)

V = {v₁, v₂, …, v_n}

(2)

The second step is to construct a fuzzy matrix represented by R to express the set of a single indicator in the set and its membership. The variable r_ij can be calculated by Equation (4), where X_max represents the highest value of different indicators and X_min represents the lowest value.

$$R=({r_{\mathit{ij}})}_{\mathit{mn}}=\left[\begin{array}{ccc}{r}_{11}& \cdots & r_{1n}\\ ⋮& \ddots & ⋮\\ r_{m1}& \cdots & r_{mn}\end{array}\right]$$

(3)

$$r_{\mathit{ij}}=\left\{\begin{array}{c}{(X}_{ij}-X_{min})/(X_{max}-X_{min})\\ {(X}_{max}-X_{ij})/(X_{max}-X_{min})\end{array}\right.$$

(4)

The entropy method is used to construct the weight that affects the final evaluation result. With 35 indicators and 5 evaluation levels, the entropy and weight of indicators are defined as follows:

$$H_i=-K{\sum }_{j=1}^n{f}_{ij ln{f}_{ij}}$$

(5)

$${\omega }_i=\frac{1-H_i}{m-{\sum }_{i=1}^m{H}_i}$$

(6)

And K = 1/ln5, f_ij = ${\mathrm{r}}_{\mathrm{i}\mathrm{j}}/{\sum }_{\mathrm{j}=1}^{\mathrm{n}}{\mathrm{r}}_{\mathrm{i}\mathrm{j}}$ (I = 1, 2, …, 35)

An indicator with a smaller entropy value provides more useful information, which means a greater impact on the evaluation result and a higher weight setting. Conversely, an indicator with a bigger entropy value provides less useful information, which means less impact on the evaluation result and a lower weight setting.

The fuzzy comprehensive evaluation model is constructed to comprehensively reflect the result of each indicator, and the evaluation result is represented by B. According to the weight ω and fuzzy matrix R, the model is constructed as follows:

B = ω_i′R

(7)

3. Results

3.1. Results of the Survey

Since the implementation of the sponge city project, both project cities have received significant feedback. Comprehensive treatment methods have been implemented such as rebuilding urban drainage networks, restoring urban natural water body functions, and improving urban sewage treatment capacity. Although resource-based water-scarce cities and poor-water-quality water-scarce cities have different water environment problems, the effect of urban water environment governance has been significantly improved. The average value of various indicators is shown in

Table 3. Average value of evaluation indicators.

Indicator	S City	C City	Indicator	S City	C City	Indicator	S City	C City	Indicator	S City	C City	Indicator	S City	C City
Q1	1.68	2.13	Q8	2.12	1.83	Q15	1.67	2.50	Q22	1.63	1.20	Q29	3.12	2.33
Q2	1.57	1.67	Q9	1.87	1.67	Q16	1.89	2.33	Q23	1.35	1.30	Q30	1.84	2.50
Q3	1.33	1.37	Q10	1.87	2.67	Q17	2.47	1.67	Q24	2.67	2.00	Q31	2.47	2.00
Q4	1.53	1.83	Q11	2.24	1.50	Q18	2.13	1.70	Q25	2.73	2.50	Q32	3.32	1.83
Q5	1.72	1.50	Q12	1.72	2.33	Q19	1.86	1.90	Q26	2.13	2.33	Q33	2.37	2.30
Q6	1.63	2.33	Q13	1.76	1.33	Q20	2.24	2.13	Q27	2.83	2.33	Q34	2.68	1.67
Q7	1.72	1.67	Q14	2.89	3.00	Q21	1.73	2.67	Q28	2.13	2.67	Q35	2.53	1.67

. Standardized processing of the data is to obtain the entropy and weights of 35 specific indicators in this policy evaluation framework (see

Table 4. Entropy and weight of each evaluation indicator.

Indicator	Entropy	Weight	Indicator	Entropy	Weight	Indicator	Entropy	Weight
Q1	0.813	0.184	Q13	0.864	0.133	Q25	0.952	0.036
Q2	0.834	0.161	Q14	0.971	0.032	Q26	0.947	0.042
Q3	0.932	0.066	Q15	0.857	0.143	Q27	0.932	0.067
Q4	0.825	0.175	Q16	0.836	0.178	Q28	0.947	0.042
Q5	0.837	0.088	Q17	0.921	0.073	Q29	0.935	0.056
Q6	0.867	0.128	Q18	0.903	0.098	Q30	0.924	0.075
Q7	0.937	0.063	Q19	0.928	0.061	Q31	0.953	0.038
Q8	0.974	0.025	Q20	0.971	0.024	Q32	0.932	0.047
Q9	0.946	0.051	Q21	0.908	0.083	Q33	0.878	0.122
Q10	0.961	0.032	Q22	0.825	0.174	Q34	0.892	0.097
Q11	0.968	0.027	Q23	0.866	0.129	Q35	0.913	0.076
Q12	0.863	0.133	Q24	0.888	0.142

). The different dimensions of comprehensive evaluation indicators at different stages are shown in

Table 5. Indicators of policy evaluation at different stages.

Policy Evaluation	S City	C City
Policy formulation	1.457	1.539
Policy implementation	1.596	1.673
Policy results	1.652	1.614

.

3.2. Evaluation of Policy Formulation

According to the empirical results, it can be seen that the gap between the two cities in the evaluation of the policy formulation stage is not that much. The policy system of the S city is more complete, and the policy objectives formulated by the C city are more feasible.

In the early stage of the construction of the project in S city, a special plan for sponge city construction and ecological comprehensive renovation of the designated river section was completed by 2020. A series of government policy documents such as sponge city planning and management regulations, sponge city construction project review, construction drawing review and special acceptance procedures have been promulgated, forming a closed-loop management system for the whole process. By 2022, the city’s sponge city construction management regulations will be issued as local regulations, with a comprehensive policy system coverage (Q9). The main responsibilities of the municipal departments are clarified in the implementation opinions on promoting sponge city construction in the whole region (Q3). As for the C city, it issued sponge city construction and management regulations at the early stage of the project construction, which put forward the corresponding standards for the planning and design objectives. The target control rate of the total annual runoff in the area is determined as 80% with 32.5 mm rainfall, and the stormwater prevention target is to effectively cope with rainstorms of not less than 30 years in the central urban area (Q1). A relatively specific technical guidance scheme (Q4) is provided, and a clear technical standard and reference basis (Q6) is set up for the subsequent project construction.

While the policy formulation of sponge city projects has shown some positive results in these two cities, some variations can be attributed to factors such as economic development level, size of the project and local laws and regulations may lead to different results. When local governments lack regulations about necessary procedures and related technology, the objective of the sponge city project may become short-term, without advanced techniques, resulting in unreasonable process schemes and failure for the construction of suitable stormwater management and improvement of local water ecology.

3.3. Evaluation of Policy Implementation

There is also little difference between the two cities in the evaluation of the policy implementation stage. The resource allocation, professional knowledge sharing and overall arrangement capacity of policy implementation in the S city are much prominent. In the C city, the ecological identification qualification of policy implementation is better, and the construction of low-impact facilities is relatively comprehensive.

The special sponge city construction leading group set up in S city is responsible for communicating and coordinating the work among different government departments (Q11). At the same time, the sponge city construction management service center is set up in the Municipal Housing and Construction Bureau, effectively practicing the construction needs of sponge city from the aspects of construction planning drawing review, site construction permit issuance, facility quality supervision, and completion acceptance (Q12). In order to enhance the scientific and forward-looking construction of sponge city, City C invested 16 million yuan at the initial stage of the project to set up a think tank with a research team from a key university as the main body, responsible for the overall consulting planning, construction site inspection, completion acceptance and maintenance management of the project (Q10). Through comprehensive mapping, inspection and restoration of urban drainage networks and water environment pollution sites, the design drainage capacity of pipelines (Q15) is restored. Targeted renovation and renovation programs (Q18) are applied to more than 40 waterlogging points and more than 10 overflow outlets in urban areas. Vertical site low-impact facilities are constructed in urban parks, considering water storage space and surrounding residential as a whole to carry out diversified rainwater resource utilization (Q19).

While the policy implementation of sponge city projects has shown some positive results in these two cities, some variations can be attributed to factors such as urban space and the ability of project professionals may lead to different results. In cities with less spare land, project implementation might face more challenges due to funding constraints, like the restriction of building new green infrastructure and revitalizing existing infrastructure in a safe and environmentally friendly way.

3.4. Evaluation of Policy Results

The evaluation scores of cities in the two stages of policy formulation and policy implementation are similar. It can be found that although the hydrological characteristics and water shortage conditions of cities are different, the sponge city project fully plays the role of different actors, uses different resources available, and alleviates the contradiction of urban stormwater and drought under reasonable rules. The ecological function of urban natural water bodies is restored and the effect of water environment governance is realized. The following is a detailed analysis of the policy results based on the actual construction of sponge city projects.

The project of S City focuses on improving the urgent situation of urban water shortage and the aging system of the drainage network. It has constructed a sustainable stormwater management system through multi-site reconstruction projects such as pipelines, roads, plazas and old residential areas. According to the actual construction needs, the reconstruction project is divided into the north-south river interception main pipe and the road rain and pollution diversion reconstruction project. The river trunk transformation was changed into 28.8 km, of which the length of the railway protection was 0.95 km, with 1 storage pond, and 9 rainwater pumping stations. The sewage pipe network was shortened to 29.8 km and the rainwater pipe network of 45.8 km. Through activating the existing drainage infrastructure, the project reduces the amount of new construction of gray infrastructure. Rational utilization of urban space and original natural waters would improve economic performance and bring ecological benefits. The city’s important traffic hub road “Ziqi Road” demonstration transformation has installed a slow greenway system, landscape green planting and other different forms on both sides of the 9.7 km road to improve soil permeability in permeable paving. At the square in front of the high-speed railway station, a newly constructed wetland is built for regulation, storage and purification of rainwater, so as to realize the recycling and reuse of irrigation and landscape water. This wetland has achieved a control rate of 85% of the total annual runoff within 0.12 km² and the annual rainwater utilization rate has exceeded 20,000 tons.

In particular, S City builds a comprehensive intelligent water ecological management platform. Due to the numerous roles of actors and complex problems, the project needs a large amount of resources to be mobilized. Various government departments, private sectors, third-party professional institutions and the public are all involved in the construction of the project, which requires a lot of overall planning and coordination work. Also, manual management is prone to problems such as multiple security risks, high cost and low efficiency. Thus, it is indispensable to construct the intelligent management system engineering of urban drainage collection and treatment facilities. This comprehensive platform currently includes flood prevention and control, pipe network management, water quality monitoring and sponge project construction in four parts. The flood prevention and control part is responsible for warning, simulation exercises and alarms in pre-flood season, personnel scheduling and rescue in flood season. The pipe network management is mainly based on the operation and maintenance of the dispatching unit named “source-network-plant-river”. The water quality monitor part is responsible for regular monitoring of urban waters. Sponge project construction includes project management, performance evaluation and other related content. At present, the monitoring terminal is mainly set in water quality monitoring levels, sewage networks, rainwater stations, sponge basements and other key locations. The terminal synchronizes the groundwater dynamic monitoring database to the platform to realize the joint urban construction and sharing of hydrology, environmental protection and other aspects of information.

The project of C City focuses on improving the quality of urban water bodies, restoring their ecological functions, and improving the overall water environmental benefits. The water quality of black and smelly water bodies is purified, and the self-purification ability of water bodies is improved. For Guanhu Lake, Zhaowei and other black and odorous water bodies, the project has designed remediation plans to make them free of black moss, eutrophication and large-scale blue-green algae outbreaks, to improve water quality transparency. More than 10,000 m³ of sediment was removed from the lake bottom, 0.4 hectares of aquatic plants were replanted, and 18,000 m³ of rainwater storage space was increased. It has become an important habitat for wild birds such as ducks and egrets. The scale of ecological management projects has expanded, including two constructed wetlands of a total of 0.25 km², ecological slope protection along the rivers of a total of 0.27 km², an ecological water re-injection project and 14 hydrophilic landscape platforms. The fragile ecosystem along the river was treated to form the water environment protection pattern of the main river, the point source pollution in the environment was effectively controlled, the self-purification capacity of the water body and the quality of the water landscape was stabilized, the conservation capacity of the urban water ecology was improved, and a healthy and safe water ecosystem was built.

Moreover, C city builds a science popularization center for the sponge city project to enhance the public’s sense of participation and gain in the management of the urban water environment. A series of products with “one museum, two centers, four parks and five stations” as the main body has been actively constructed, attracting urban residents to enter and build a sponge city scientific system in an interactive way. The public not only gains knowledge of sponge city construction, and understands its function and significance, but also makes the wetland park a first-class wetland science popularization, research and exhibition center. The construction of this venue and corresponding facilities has increased the enthusiasm of the public, expanded the knowledge-sharing channels of water ecology, and stimulated the citizens to participate in the activities of urban water environment protection in their daily lives. The planning and construction of urban ecological facilities produce the value of service supply after being put into use, realize the governance effect of urban ecological services in the linkage interactive experience, and enhance the cultural connotation of sustainable urban water environment management.

While sponge city projects have shown positive results in these cities, variations in effectiveness can be attributed to factors such as city size and geological conditions. Smaller cities are faced with a shortage of continuous and stable investment. With complex and varied terrain conditions, the results of the projects may need a longer period to realize, requiring more suitable solutions and advanced technology.

4. Discussion

4.1. Discussion for Policy Formulation

These two projects integrate past fragmented urban ecological services to the continuous unity of the whole by way of inter-departmental synergies. Through the means of market competition, social capital represented by the private sector enter the supply structure and improve efficiency [57]. However, the government and private sector have conflicting goals with interdependent interests could make conditions more complex for policy formulation. The goal of the government is to maximize the public interest, while the goal of the private sector is to maximize the profit. The conflicting goals of such cooperation make the government assume the role of regulator in order to prevent the private sector from harming the public interests. Thus, strategic synergy becomes a suitable form of collaboration between the government and the private sector [58]. The projects have built a cooperative relationship and utilized institutionalized cooperation mechanisms for policy formulation.

4.2. Discussion for Policy Implementation

Based on the current hierarchical structure of sponge city projects, the policy is mainly implemented by professional departments, guided by the government, with a small amount of social participation. In the long term, it is necessary to continue the steady investment of funds, personnel management and operation and maintenance. So, the future sponge city construction could be improved in the following direction: it is important to make sponge city a long-term project through comprehensive policy implementation, enhancing the authority and credibility; to increase the weight of follow-up operation, maintenance and supervision of ecological infrastructure in the assessment system, ensuring that the facilities reach the target service life after acceptance; to improve the follow-up supervision and management mechanism of funds and personnel. Thus, a co-governance model could be developed, and the systematic and comprehensive promotion of sponge city would then be realized in the future.

4.3. Discussion for Policy Result

Faced with different hydrological conditions, projects would utilize specific techniques to solve their problems. Projects in City S focus on upgrading drainage systems to improve urgent situations in urban areas, mainly using a combination of green roofs and permeable pavements with a comprehensive intelligent platform to control and manage for stability, proving much effectiveness in terms of rainwater interception, conservation and reuse. By contrast, projects in City C take measures to improve urban water quality, mainly by growing beneficial plants in waters, forming ecological protection areas, and constructing a science popularization center, which attracts citizens to actively participate in and expand the influence of the results of the projects.

The social ecosystem can not only realize the value shaping of the interaction between humans and nature but also formulate the social framework of interaction through the urban geographic information system to achieve urban water ecological benefit [59]. Also, green infrastructure can help social groups become participants and managers in the provision of ecological services. Expanding the sharing of expertise is an important way to maintain the results and enhance the influence of the projects. Using diversified online and offline channels, the sponge city information-sharing mechanism would be constructed to realize the creation and sharing of ecological knowledge in the whole society. A variety of sponge city publicity activities would be set up in the science popularization center to provide space for city residents to form a concrete understanding and enhance interest. It is a good choice to build a demonstration visiting and learning community, develop a sponge city practice education base in the urban green space, and provide social support for the sustainable development of the urban water environment.

5. Conclusions

This study demonstrates that sponge city projects can significantly enhance urban water management, particularly in water-scarce regions. By mimicking natural hydrological processes, these projects not only reduce flooding but also promote sustainable urban development. Government-led sponge city projects can help water-scarce cities build sustainable stormwater management systems and control urban water ecological problems. Reconstruction of drainage pipe networks and diversion of rainwater and pollution would effectively improve the level of urban stormwater management [60]. The construction of ecological wetlands and treatment of original urban waters enhance urban rainwater reserves. It is crucial to conserve and restore the ecological functions of urban waters and utilize sustainable green infrastructure to achieve the goal [61]. Moreover, with high adaptability, the sponge city project can customize its own technical solutions and carry out effective implementation according to the city’s hydrological characteristics and specific water environment problems. In order to achieve sustainable management of the urban water environment and better meet public demand, the urban aquatic landscape pattern can be designed and planned appropriately. Different subjects can realize interdisciplinary action cooperation, and carry out adaptive transformation for different governance units, so as to balance urban ecological needs with social values like aesthetics, comfort and safety. In-depth expertise can be gained in practice and be used in the overall planning of subsequent sponge city projects.

Sponge city projects contribute effective new construction strategies for different types of water environment problems in water-scarce cities. For similar policy evaluation, comprehensive evaluation can be achieved from both the time dimension (multi-level policy process) and geographical dimension (water-scarce cities with similar hydrological conditions). Also, the policy evaluation model can deal with fuzziness and uncertainty to a certain extent and use quantitative methods to incorporate unquantifiable indicators into the evaluation framework. However, policy formulation does not have a direct causal relationship with the perception of policy results and may affect policy implementation to a large extent. Policy implementation has the greatest weight on policy results. The lack of a direct relationship between policy formulation, policy implementation, and policy result scores can be explained by higher stakeholder expectations of policy results, differences in respondents’ perception levels and knowledge and unexpected gaps between hypothetical policy scenarios and reality. Future research could adjust the evaluation system to sponge city projects with more complex conditions to optimize the effect and explore the direct causal relationship between policy formulation, policy implementation and policy results to maximize their environmental and social benefits based on a larger extent of data sample.